Time-division multiplexing is a widely used method of transmitting a number of data signals supplied by various sources over one transmission path. Briefly, this method divides the time available into repetitive frames of equal length and each frame is in turn divided into a number of time slots. Each of these is allocated to one data source and is used to transmit either a single bit or a multibit character, depending upon the type of multiplexer involved. In order to detect the positions of the various time slots within a given frame, the multiplexers must be able to determine where the frame begins. Accordingly, the first time slot is used for synchronization purposes. In a character-by-character type of multiplexer, the first time slot always contains a known reference character called a frame-alignment character, which must be received by the remote multiplexer, or demultiplexer, at the beginning of at least two successive frames before transmission of the data can begin. In a bit-by-bit multiplexer, the first time slot is successively used to transmit the different bits of a frame-alignment character and the transmission of data can only begin after the frame-alignment character has been correctly received by the demultiplexer.
The various time-division multiplexing methods and devices that are employed to realize relatively simple multiplexers, such as the IBM 2712 Multiplexer, or more sophisticated ones, like the IBM 3705 Multiplexer, have been extensively described in the literature. However, all of these methods and devices, while suitable for implementing such multiplexers, which are transparent to the signals to be multiplexed (i.e., operate without regard to the nature of the signals involved), do not lend themselves to the realization of simpler devices capable of combining signals of different types, such as data signals and secondary signals, for transmission over a single transmission path.
The need to transmit data signals and secondary signals arises in a teleprocessing network. In such a network, a master data-terminal equipment (DTE) communicates with several slave DTE which lie remote therefrom. The master DTE communicates with each remote DTE through an associated data circuit-terminating equipment (DCE), known as a modem, and through a DCE associated with the remote DTE. The "Comite Consultatif International Telegraphique et Telephonique" (CCITT) has defined in its Recommendation V24 the interface between a DTE and the DCE associated therewith. This interface includes the binary interchange circuits necessary for the transfer of binary data, control and timing signals. To connect a DTE to a DCE, a 16-conductor cable is generally used to transfer sixteen of these binary signals. The cable has a limited length so that specialized modems must be used to connect the master DTE to the associated DCE whenever the distance between the two units exceeds the maximum length of the cable. Also, where the DTE is a multiplexer that is to be connected, for example, to one hundred DCE's, one hundred of said cables are required and the exterior size of the DTE must be such as to enable the physical connections to be made. Large scale integration techniques make it possible to achieve a very substantial reduction in the size of the electronic circuits within the DTE. Thus, the physical exterior is much larger than the necessary interior. It is therefore desirable to provide a means of transmitting the V24 interface signals over a transmission path that does not require the use of sixteen conductors.
French patent application No. 78 29352 filed by the present applicant describes a method of simultaneously encoding first and second sequences of binary signals as one sequence of pulses to be transmitted over a transmission path. The first sequence consists of the data provided by the DTE. The second sequence results from the time-division multiplexing of the control signals supplied by the DTE. In accordance with the method described in the aforesaid patent application, the binary signals are paired off, with each pair being comprised of binary signals belonging to the first sequence and to the second sequence, respectively. Each pair is then encoded as follows: if the binary signal from the first sequence is at a first logic level, the pair is encoded as a bipolar signal; if the signal from the second sequence is at a second logic level, the pair is encoded as a two-phased signal.
The above method, among many others, can be used to provide an interface comprised of a transmitter and a receiver for transmitting signals from the V24 interface over a transmission line; however, its disadvantage is that it does not allow the data signals to be sent at more than one bit rate. Since a given DTE can usually transmit data at several bit rates, one of said interfaces comprised of a transmitter and a receiver would have to be provided for each bit rate together with means for detecting any change in the bit rate and for selecting the appropriate interface accordingly.